Making the on method of engine generic

When subscribing to events directly from rules instead of the success and failure events the on method
has the wrong signature for the event handler make it generic to fix it.

Replaces CacheControl#370
Co-Author Michael J. Shalewski <mshalewski@outlook.com>

Author: chris-pardy

types/index.d.ts

@@ -51,9 +51,7 @@ export class Engine {
   removeFact(factOrId: string | Fact): boolean;
   getFact<T>(factId: string): Fact<T>;
 
-  on(eventName: "success", handler: EventHandler): this;
-  on(eventName: "failure", handler: EventHandler): this;
-  on(eventName: string, handler: EventHandler): this;
+  on<TEvent = Event>(eventName: string, handler: EventHandler<TEvent>): this;
 
   run(facts?: Record<string, any>, runOptions?: RunOptions): Promise<EngineResult>;
   stop(): this;
@@ -119,8 +117,8 @@ export interface Event {
 
 export type PathResolver = (value: object, path: string) => any;
 
-export type EventHandler = (
-  event: Event,
+export type EventHandler<TEvent = Event> = (
+  event: TEvent,
   almanac: Almanac,
   ruleResult: RuleResult
 ) => void;

types/index.test-d.ts

@@ -4,11 +4,13 @@ import rulesEngine, {
   Almanac,
   EngineResult,
   Engine,
+  Event,
   Fact,
   Operator,
   OperatorEvaluator,
   PathResolver,
   Rule,
+  RuleResult,
   RuleProperties,
   RuleSerializable
 } from "../";
@@ -94,6 +96,19 @@ expectType<Engine>(engine.addFact(dynamicFact));
 expectType<boolean>(engine.removeFact(fact));
 expectType<Fact<string>>(engine.getFact<string>("test"));
 
+// event handlers
+engine.on("failure", (event, almanac, ruleResult) => {
+  expectType<Event>(event)
+  expectType<Almanac>(almanac)
+  expectType<RuleResult>(ruleResult)
+});
+
+engine.on<{ test: true }>("test", (event, almanac, ruleResult) => {
+  expectType<{ test: true }>(event);
+  expectType<Almanac>(almanac);
+  expectType<RuleResult>(ruleResult);
+});
+
 // Run the Engine
 expectType<Promise<EngineResult>>(engine.run({ displayMessage: true }));